Systems for asynchronous communication between electronic devices which generate and receive digital signals, such as power system protective relays but also including other electronic devices such as computers, etc., are well known. In asynchronous communication, a transmitting electronic device will typically produce a ground referenced signal for communication with another such device. That signal is often transmitted over a metal cable to a receiving device, which measures the received signal against ground. Asynchronous communication typically uses the EIA-232 ASCII communication format. The internal clocks of the transmitter and the receiver are not synchronized in asynchronous communications.
The metal cable has a certain capacitance. If the data communication has a maximum Baud rate of 38.4 kBaud, the typical maximum distance between a transmitter and receiver will be less than 50 feet. This is disadvantageous in those circumstances, such as in power systems, where the distance between two electronic devices will be significantly greater than 50 feet. For instance, nine miles (15 km) may be a typical distance between electric power substations in concentrated load centers, while in many cases, the distance between two substations will be significantly greater than nine miles and in some cases greater than 50-70 miles.
In many applications, such as between two power system protective relays, high security and reliability are required for the transmission of data. An example of such data communication where security and reliability are important requirements is the transmission of information relative to whether the location of a fault is on a power transmission line between two specific protective relays or is on some other transmission line.
Metallic cables, however, in conventional communications systems, have several disadvantages, including safety problems which can occur during electrical faults. Also, there will typically be a substantial increase in the bit error rate (BER), which results in communication of data becoming unreliable, during such faults. Further, metallic cables are susceptible to interference associated with electrical disturbances such as lightening which may result in momentary or permanent damage to the communications electronics.
Fiber-optic cables have become the preferred method of communication where high security and reliability is required. Fiber-optic cables in addition provide the desired electrical isolation which eliminates or significantly reduces safety concerns with metallic cables and eliminates or significantly reduces the increase in BER during faults. On the other hand, fiber-optic cables have some disadvantages, most of which concern power requirements. The attenuation of fiber-optic cable is known, depending upon the characteristics of the cable. A significant objective relative to fiber-optic cable transmission is to maximize the optical power budget, which is calculated according to a known formula, in order to provide the greatest distance capability with the least power.
High power lasers are frequently used in fiber-optic communication systems to achieve long distance communication. Such lasers, however, are typically large, expensive, require significant electrical power to operate, and pose a safety hazard to the eyes and skin of the operator. In addition, if it is desired that the transceiver obtain its power from the electronic device and hence be mountable to the electronic device, high powered lasers cannot be used.
A low power laser known as the Vertical Cavity Surface Emitting Laser (VCSEL), while closer to desirable power output loads, and while in addition having a desirable small size, still has an optical power output which exceeds recognized eye safety limits. It is important and desirable for the present invention to provide a transceiver which operates below the recognized eye safety limits.
There are also disadvantages of currently available optical receivers used in long distance fiber-optic communication. Such receivers should be as sensitive as possible, such as for instance APD (avalanche photodetector), or a pin photodiode. An APD, however, would not be an appropriate choice because of the requirements of large negative bias and cooling. A pin photodiode can be used, although such configurations usually require that the pin diode be reverse biased and configured in the photoconductive mode, which can lead to damage or the amplifiers being driven to saturation if too strong a signal is receiver. In such cases, attenuation jumpers are typically provided in the fiber-optic line, which is undesirable. Hence, a transceiver for fiber-optic communication is needed which meets existing eye safety standards yet has enough power and sensitivity to communicate over a variety of distances, including short distances and long distances, without the need for attenuation jumpers.